platforms/drm: Don't release drm buffers in EglGbmBackend

When we release the buffers that also removes the drm fbs.
This causes failing atomic commits that the atomic code
doesn't (yet) handle correctly, the result is a black
screen when compositing is restarted.

src/plugins/platforms/drm/drm_output.cpp

@@ -74,14 +74,6 @@ void DrmOutput::teardown()
     //this is needed so that the pageflipcallback handle isn't deleted
 }
 
-void DrmOutput::releaseBuffers()
-{
-    m_crtc->setCurrent(nullptr);
-    m_crtc->setNext(nullptr);
-    m_primaryPlane->setCurrent(nullptr);
-    m_primaryPlane->setNext(nullptr);
-}
-
 bool DrmOutput::hideCursor()
 {
     return drmModeSetCursor(m_gpu->fd(), m_crtc->id(), 0, 0, 0) == 0;

src/plugins/platforms/drm/drm_output.h

@@ -44,7 +44,6 @@ public:
 
     ///queues deleting the output after a page flip has completed.
     void teardown();
-    void releaseBuffers();
     bool showCursor(DrmDumbBuffer *buffer);
     bool showCursor();
     bool hideCursor();

src/plugins/platforms/drm/egl_gbm_backend.cpp

@@ -69,7 +69,6 @@ void EglGbmBackend::cleanupFramebuffer(Output &output)
 void EglGbmBackend::cleanupOutput(Output &output)
 {
     cleanupFramebuffer(output);
-    output.output->releaseBuffers();
 
     output.buffer = nullptr;
     output.secondaryBuffer = nullptr;